There are many applications in which it is desirable to split a signal for use by more than one portion of a system. For example, it is sometimes desirable to provide the same local oscillator (lo) frequency output to the receiver and transmitter of a communication system. Similarly, it may be desirable to provide the same output including information on a number of channels to different receivers tuned to different channels.
In such applications, it is generally desirable for each of the outputs produced by the power splitter to exhibit a matched phase and equal power. In addition, the voltage standing wave ratio (VSWR) at any port of the splitter should be relatively low when all other ports are properly terminated. Further, the power splitter should generally have a low insertion loss and should also provide isolation between all output ports and provide reverse isolation between the input and output ports. These properties should further be achievable over a broad bandwidth without loss of gain.
Several techniques have been used to provide the desired signal division and phase matching. The first such approach to be discussed is often referred to as a "lumped" approach because it relies upon the use of discrete components. For example, a lumped system may employ transformers formed by multifilar wire wound through cylindrical ferrite cores. The input signal to be divided is then applied across the primary winding of the transformer and the outputs are tapped off the secondary windings.
The lumped approach is typically limited to VHF and UHF applications, because of the interline capacitances involved. Further, the construction of such transformers is labor intensive. In addition, traditional lumped approaches sometimes offer limited phase match if transformer symmetry is not maintained.
Another conventional approach used to achieve power splitting is known as the "distributed" approach. In this configuration, the components relied upon to perform the desired signal division and phase matching are effectively distributed throughout a section of transmission line. More particularly, the input signal is applied to one end of a plurality of quarter-wavelength transmission elements that act as distributed transformers. The other ends of the transmission elements terminate, for example, in impedances to provide the desired outputs.
Because the distributed approach relies upon one-quarter-wavelength elements to, in part, achieve the desired phase matching, the components used are typically relatively large (at the frequency of interest) and restricted to an octave bandwidth. This approach does, however, absorb output load mismatch. Also, because the losses are limited to the line losses, the distributed approach exhibits relatively low insertion losses.
A third approach to power splitting is known as the "active" approach. This technique employs at least some active components and may be implemented using elements of the distributed approach. An example of such a configuration is disclosed in U.S. Pat. No. 4,769,618 (Parish et al.), where field-effect transistors are employed to provide gain between distributed points in an input transmission line and distributed points in a plurality of output lines. Although active approaches typically have a relatively broad bandwidth, they use a large number of gain stages and require cascading to produce more than two outputs.
As will be appreciated from the preceding remarks, it would be desirable to produce a relatively simple power splitter that provides equal power, matched phase outputs to a large number of ports, while having a low insertion loss and providing the isolation required between all ports.